Television receiver



Oct. 30, 1951 A. COTSWORTH, lll

TELEVISION RECEIVER Filed June 18, 1949 0 omgm o 2 SHEETS-SHEET l ALBERTCOTSWORTHID: I

IN V EN TOR.

HI S ATTORNE O B ED O.

Q 1951 A. COTSWORTH, 11x

' TELEVISION RECEIVER 2 SHEETS-SHEET 2 Filed June 18, 1949 (spqgqaq)asuodsag E H w W S T O T R E B L A (s aqgoaq) esuodsag E N R O W A m HPatented Get. 30, 1951 Aiher t Cotsvvorth, Oak Park 111., assig'nor'toZenith Radio Corporation, a corporation of Illinois Application June 18,1949, Serial No. 99,953

4 Claims.

This invention relates to television receivers and more particularly toa television receiver in which an automatic control circuit is utilizedto enable the receiver to reproduce the ima e-intelligence contained intelevision signals of a wide range of signal strengths.

In present-day television systems, the principle of vestigial sidebandtransmission is used so that the channel requirements of the transmittedsignal may be as narrow as possible. In such a signal the videocarrier-wave has but a single sideband except for the sidebands in closepro);- imity with the carrier-wave which represento'ertainlovv-frequency components of the video sige nal, and as to thesecomponents both the upper and lower sidebands are included. It isnecessary, therefore, that a television receiver utilizing such a signalbe so constructed that its intermediate-frequency amplifier amplifiesthe television signal with its double sideband portion attenuatedrelative to its single sidebandportion in order to produce the effect ofequal response to all components of the picture signal, so that theimage represented by these components may be reproduced by the receiverwith full picture detail. To accomplish this, the intermediate-frequencyamplifier is made frequency selective and has a response characteristicwith an intermediate portion of substantially uniform maximum responseand an end portion of die creasing response. The amplifier is soconstructed that the video intermediate-frequency carrier lies in thisend portion, and this carrier and its double sideband components arethereby translated by the amplifier attenuated relative to the singlesideband components. This results in insufficient amplification of thecarrier and the receiver is capable of reproducing only relativelystrong'received signals.

The present invention provides a television receiver which utilizes anautomatic control circuit to vary the width of the uniformmaximurh'response portion of the amplifier characteristic inversely withthe strengths of the received television signals. The circuit is soarranged that the intermediate-frequency carrier-wave of any receivedtelevision signal above a certain predetermined strength lies in thedecreasing response end portion of the receiver characteristiq'as in 2weal; signal lies in the maximum response portion of the characteristicand not in the end portion as was the case for the stronger signals.

In this manner, signals below the predetermined strength have theircarriers translated by the amplifier with substantially "no attenuationrelative to its single-sideband components and theoverall response ofthe receiver to these weak signals is materially increased. In thiscondition, the single-sideband components have the effect of beingattenuated relative to the double sid'eband components of the translatedsignal since the amplifier responds equally to all components of thesignal, and'sofne detail is lost the reproduced image. However, when thereceived "signal weak and 'the signalto noise ratio is low, the improvedeffective signal-tonoise ratio realized by the action of the automaticcontrol circuit more than compensates for the loss'in picture 'detail inthe reproduced image. It is accordingly an object of this invention toprovide animproved television receiver capable of reproducingsatisfactory images from the intelligence contained in televisionsignals of a wide range of signal strengths.

Another object of this invention is to provide an improved televisionreceiver which utilizes an automatic control circuit to alter thecharacteristics of the receiver in response to the signal strengths ofthe received signals to enable the' receiver to reproduce satisfactoryimages from the intelligence contained in relatively weaktelevisionsignals to which the receiverwould otherwise be insensitive.

The features 'of this invention which are believed to 'be new are setforthwith particularity in "the appended claims. The invention itself,however, together with further objects and advantages thereof, may bestbe understood by reference to the following description when taken inconjunction with the accompanying drawing in which Figure 1 shows atelevision receiver incorporating 'the automatic control circuit of theinveniq n 'Fi'gure'sfl and 3 show various curves useful in theunderstanding of the invention.

The television receiver of Figure 1 includes the usual radio-frequencyamplifier ill, 'first detector Il-and first and second videointermediate-frequehcy amplifiers I2 and is. The radio-frequencyamplifier it may be connected to a suitable antenna 14, and the outputterminalsof'the intermediate-frequency amplifier l3 are coupled to *athird intermediate-frequency amplifier t5.

The output circuit of intennediate-frequency amplifier I is connected toa video detector l6 which, in turn, is coupled to a video amplifier H.The output terminals of the video amplifier I! are connected to thecontrol electrode and cathode of an image reproducing device l8.

The afore-described portion of the receiver, except for amplifier stageI5, is quite conventional. The radio-frequency amplifier IO and thefirst detector may be tuned to receive and amplify a television signalintercepted by the antenna M. This signal is amplified in theradiofrequency amplifier II], which may have one or more stages, and isheterodyned to the selected intermediate frequency of the receiver inthe first detector The resulting intermediate-frequency signal isamplified in the intermediatefrequency amplifier stages l2, l3 and I5,three stages of intermediate frequency amplification being shown merelyby way of example. The amplified intermediate-frequency signal from theamplifier I5 is detected in the video detector I6 and the resultingvideo signals are amplified in the video amplifier H, which may have oneor more stages. The amplified video signals are supplied to the devicel8 to control the intensity of the electron beam therein in well-knownfashion.

The synchronizing system and sound system of the television receiverform no part of the present invention and for that reason have not beenshown.

The intermediate-frequency amplifier |5 embodies the present inventionand includes an electron-discharge device 20. One of the outputterminals of the intermediate-frequency amplifier |3 is connected toground, and the other is coupled to the control electrode 2| of thisdevice through a coupling capacitor 22. The cathode 23 of the device 20is connected to ground through a resistor 24, this resistor beingbypassed by a capacitor 25. The screen electrode 26 of the device 20 isconnected to the positive terminal B+ of a source of unidirectionalpotential, not shown, through a resistor 21, this screen electrode beingby-passed to ground through a capacitor 28. The suppressor electrode 29of the device 20 is connected to ground. The anode 33 of the device 20is connected to the positive terminal B+ of a unidirectional potentialsource through the primary winding 3| of a coupling transformer 32 and aseries-connected resistor 33. The resistor 33 is by-passed to ground bya capacitor 34. The anode 30 is also connected to the input circuit ofthe device 20 by means of a feedback network including series-connectedresistors 35 and 36. The junction of the resistors 35 and 36 isconnected through a resistor 31 to the junction of resistor 33 andcapacitor 34.

One terminal of the secondary winding 38 of transformer 32 is groundedand the other terminal thereof is connected to the cathode of aunilateral conductive device 39. The anode of the device 39 is coupledto the ungrounded input terminal of the video amplifier I! through aninductance coil 40 and by-passed to ground through a capacitor 4|. Theinductance coil 40 is connected to a second inductance coil 42 which, inturn, is connected to ground through a resistor 43. The network 4|43forms a usual filter network commonly used in video detector circuitsfor removing the intermediate-frequency component of the signal suppliedto the video amplifier [1. The junction of the coil 42 and resistor 43is connected to one of the input terminals of an automatic gain controlstage 44 through a resistor 45, the other input terminal of this stagebeing grounded. The stage 44 may be of any well-known type and developsa negative control potential having amplitude variations dependent uponthe strengths of the signals translated by the receiver.

The automatic gain control stage 44 is connected to the amplifiers I0,l2 and I3 of the receiver by way of lead 46. The lead 46 is furtherconnected to the control electrode 2| of the device 20 through aresistor 41 and inductance coil 48. The coil 48 and shunt-connectedcapacitor 48' form a tuned circuit for coupling the amplifier IE to theamplifier I3. The junction of the resistor 4'1 and coil 48 is lay-passedto ground through a capacitor 49, and the junction is further connectedto the cathode of a unilateral conductive device 5|]. The anode of thedevice 50 is connected to the negative terminal C of a source ofunidirectional biasing potential, not shown.

The negative control potential developed by the automatic controlcircuit 44 is supplied to the control electrodes of theelectron-discharge devices included in the amplifiers I0, l2, and I3 tocontrol the bias on these electrodes. In this manner, the gain of theseamplifiers i made of function of the relative amplitudes of the incomingtelevision signals to which the receiver ma be tuned.

The video intermediate-frequency amplifier I5 is made degenerative byreason of the negative feed-back circuit comprised of the resistors 35,36 and 31. Due to the characteristics of negative feedback amplifiers, achange in the bias on the control electrode 2| does not materiallyaffect the gain of this stage but does alter the bandpasscharacteristics thereof. More particularly, any increase in the biaspotential of such an amplifier decreases the acceptance band, whereas areduction in the bias has the opposite effect and increases theacceptance bandwidth. Consequently, when weak television signals arereceived by the receiver, the negative automatic gain control potentialdeveloped by the circuit 44 is of relatively low amplitude, and thiscontrol potential which is supplied to the control electrode 2| throughthe resistor 41 act to broaden the acceptance band of the amplifier l5.However, for stronger signals the negative control potential increasesand the band-pass of the amplifier I5 is accordingly narrowed.

The device 50 connected in shunt with respect to resistor 41 functionsas a limiter to limit the maximum value of the automatic-gain controlpotential applied to the amplifier l5 to a preselected level determinedby the potential source C- which, in turn, establishes a bias on thedevice. Whenever the control potential from the automatic-gain-controlstage 44 exceeds the bias from source C, the device 50 becomesconductive. Consequently, the control potential as applied to amplifierl5 cannot exceed to any appreciable amount the value which causes thelimiting device to conduct. In practice, the bias of the device 50 is sochosen that limiting occurs during reception of any signal that has atleast a medium signal strength.

Figure 2 shows the overall band-pass characteristics of theintermediate-frequency amplifier stages l2, l3, and I5 during thereception of signals above a predetermined signal strength. During thereception of these signals the device 5 50.: is. cpnductive and limi sthe-bias the con: rol electrode 2.1, f; the dev c Zfl; to arreselecteamaximum value. Under these. c nd t o s, he. ve a r sponse o the. nte medate-freq ency amp fie s a es 1-35 it. and f encv selecti e and a shareer s i sender. to.- he. char cteristic. 2 he mitts-tramway: am: lifie usal, r s I .ev telev s on rece erhe ov a l. re uenc RSI.) pres nt arec6115 sponse. a hera hat. he-vide nt r samba-frequency a ie his and e tne- Ther ora. the ideo arrier an its. dou leiemens. mpon nts; re; r nated. by. t e; amn iiers at e uated relative. to h inslersidehans comm.lor .011 izl t ict r d a he. ide 0 -wav. sn. his e po n a a nt a nremate y ne-ha he masimumresnense orthe amnl fier- Figure 3 shows. theoverall band-pass characteristics of the intermediate-frequencyamplifier stages I2, i3, and it during the reception of a televisionsignal below the predetermined signal strength. During the reception ofthis signal the device 50 is non-conductive and the bias on the controlelectrode 21 of device 29 is determined by the control potentialdeveloped by the automatic-gain-control stage 44. The bias on controlelectrode 21 is less than during reception of strong signalsand thistends to broaden the intermediate portion of theoverall; response curve60/ of the amplifiers. The video mtermediate-frequency carrier in thiscondition no longer liesin the end portion of the characteristic ofdecreasing response, but in the intermediate portion of maximum uniformresponse.

The overall response of the amplifiers in the latter condition is nolonger frequency-selective, and the video intermediate frequency carrierand all its sideband components are translated thereby with maximumamplification. The intermediate-frequency signal now produced in theoutput circuit of the amplifiers has its single-sideband components ineffect attenuated relative to its double-sideband components since thereceiver responds equally to all the components.

'As previously mentioned, this tends to distort slightly the imagereproduced by the receiver. However, the improved signal-to-noise ratioof the signal more than compensates for this effect, and satisfactoryimages may be reproduced by the device 18 from weak television signalsto which the receiver would otherwise be insensitive.

It is desirable that the action of the control potential on thecharacteristic of the amplifier I does not materially affect the overalllow frequency response of the amplifier stages l2, l3, and I5 in orderthat the sound intermediatefrequency carrier-wave is never translated bythe video intermediate-frequency channel. This may be accomplished byconstructing the amplifier stages I2, l3, and !5 with individualpredetermined staggered frequency-selective characteristics to producethe desired overall response characteristic. The frequency-selectivecharacteristics of these stages may be such that alterations in theacceptance band of the amplifier [5 to which the control is applied,alters the overall response at the high frequency end only, asillustrated in Figures 2 and 3. In this manner, the sound carrier-waveis not translated by the stages l2, l3 and i5 regardless of whethertheir overall: characteristics is a h wgir Blame 0.2 as show n Figure 3.

.ll....n entiqnnrqv d s herefo animnroved: el vis n recei rw c an auomatic, Q. trol circuit is provided that acts the presence of; Weektelevision. signals to broaden cc ptance n th video nt rme a erequency'amplifier so that the image intelligence contained therein may besatisfactorily reproduced by the receiver image tube,

While a particular embodiment of the invention has been shown anddescribed, modificar. tions may be made, and it is. intended. in theappended claims to cover all such modifications; as, fall. the, truespirit and scope of the. invention.

1,, A television. receiver for. utilizing modulated.-. carriertelevision signals, of the vestigial side; band type subject to av widerange of Signal strengths. comprising: a frequency-selective amplifierhaving a response characteristic with an intermediate portion ofsubstantially uniform maximum respons and an end portion of de-.-creasing response said amplifier including an. electron-discharge.devicehaving input and out-: put circuits and further including adegenerative; feedback network coupled between said input and outputcircuits; a control system for developing a control potential varying inamplitude with variations in the strength of the receivedtelevisionsignals; a biasing network coupled be.- tween aid now; ircui dsaid control s tem or pn vin sai contro po tia o i p fier to vary thewidth of said intermediate portion of said characteristic inversely withthe amplitude of said control potential; and a limiter for limiting saidcontrol potential as applied to said amplifier to a preselected maximumvalue for which the Width of said intermediate portion is such that thecarrier component of any one of said received television signals above apredetermined signal strength lies in said end portion.

2. A television receiver for utilizing modulated- 7 carrier televisionsignals of the vestigial sideband type subject to a wide range of signalstrengths comprising: a frequency-selective amplifier having a responsecharacteristic with an intermediate portion of substantially uniformmaximum response and an end portion of decreasing response, saidamplifier including an electron-discharge device having input and outputcircuits and further including a degenerative feedback network coupledbetween said input and output circuits; a control system for developinga control potential varying in amplitude with variations in the strengthof the received television signals; a biasing network coupled betweensaid input circuit and said control system for applying said controlpotential to said amplifier to vary the width of said intermediateportion of said characteristic inversely with the amplitude of saidcontrol potential; and a limiter coupled to said biasing network forlimiting said control potential as applied to said amplifier to apreselected maximum value for which the width of said intermediateportion is such that the carrier component of any one of said receivedtelevision signals above a predetermined signal strength lies in saidend portion at a point of approximately one-half of said maximumresponse.

3. A television receiver for utilizing modulatedcarrier televisionsignals of the vestigial sideband type subject to a wide range of signalstrengths comprising: a frequency-selective amplifier having a responsecharacteristic with an intermediate portion of substantially uniformmaximum response and an end portion of decreasing response, saidamplifier including an electron-discharge device having input and outputcircuits and further including a degenerative feedback network coupledbetween said input and output circuits; a detector circuit coupled tosaid amplifier for detecting the signals amplified thereby; an automaticgain control circuit coupled to said detector for developing a controlpotential varying in amplitude with variations in the strength of thereceived television'signals; a biasing network coupled between saidinput circuit and said control system for applying said controlpotential to said amplifier tovary the width of said intermediateportion of said characteristic inversely with the amplitude of saidcontrol potential; and a limiter coupled to said biasing network forlimiting said control poten'-' tial as applied to said amplifier to apreselected maximum value for which the width of said intermediateportion is such that the carrier com ponent of any one of said receivedtelevision signals above a predetermined signal strength lies ofdecreasing response, said amplifier stages individually including anelectron-discharge device having input and output circuits and at leastone of said stages including a degenerative feedback network coupledbetween said input and output circuits; a control system for developinga control potential varying in amplitude with variations in the strengthof the received television signals; a biasing network coupled betweensaid input circuit of said degenerative amplifier stage and said controlsystem for applying said control potential to said amplifier to vary thewidth of said intermediate portion of said characteristic inversely withthe amplitude of said control potential; and a limiter coupled to saidbiasing network for limiting said control potential as applied to saidamplifier to a preselected maximum value for which the width of saidintermediate portion is such that the carrier component of any one ofsaid received television signals above a predetermined signal strengthlies in said end portion.

ALBERT COTSWORTH, III.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,144,224 Koch Jan. 17, 19392,216,998 Farrington Oct. 8, 1940 2,273,639 Haantjes Feb. 1'7, 19422,289,822 Boucke July 14, 1942

